1. Field of the Invention
The present invention relates to a substrate for a display device. More particularly, the present invention relates to a fabricating method of a plastic substrate adapted to improve the stability of a display device fabrication process. The fabrication process utilizes the plastic substrate, which is inexpensive and easy to obtain.
2. Description of the Related Art
Recently, the display device market has focused its attention on flat panel display devices because these devices are easily manufactured to be thin, light and large-sized.
Flat panel display devices include, for example, liquid crystal display (LCD) devices, plasma display panels (PDP), and organic electro luminescence displays (OLED). These flat panel display devices include glass substrates. However, because glass substrates are not flexible, there is a limit in the application and use of flat panel display devices.
Accordingly, a flexible display device has been introduced as the next generation display device. The flexible display device includes a substrate of a flexible material such as plastic instead of the related art inflexible glass substrate.
Ordinarily, the plastic used for the flexible substrate is generally made of polyethersulfone (PES). This material is generally employed as the substrate for the display device because it has a high glass transition temperature (Tg).
The glass transition temperature is the transition temperature at which a polymer material changes from a glass state to a flexible rubbery state. Polymer materials, such as plastic, in their glass state, are more flexible than glass. Accordingly, polymer materials are more flexible than glass. Around the Tg, polymer materials, such as plastic, are much more flexible than when they are in their glass state. At room temperature, both polymer materials and glass are in their respective glass state, i.e. amorphous state. However, at room temperature, polymer materials in their glass state are more flexible than glass in its glass state. Accordingly, glass is harder than polymer materials at room temperature. Flexible flat panel display devices use polymer materials because they are more flexible than glass at room temperature.
Generally, display device fabrication processes are carried out at a maximum temperature of 200° C. PES has a glass transition temperature of 220° C. Because PES has a glass transition temperature higher than the processing temperature, PES is more stable against heat than other plastics that have a glass transition temperature below 200° C. The PES substrate may be prepared by the fabrication steps as shown in FIG. 1, so that it can be used in the flexible flat panel display.
Referring to FIG. 1, during the fabrication process of the display device, the surface of the PES substrate is first heat-treated as shown in step S1. This reduces future deformation that may be caused by heat generated in the array process of step S3.
In the heat treatment process of step S1, the PES substrate is deformed by heat. As shown in FIG. 2, the degree of deformity is lessened when the PES substrate is heat-treated at a temperature of 180° C. or 200° C. for more than 48 hours. The PES substrate that went through the heat treatment process (S1) has been deformed by heat. Therefore, the amount of deformation that will be caused by heat generated in the array process (S3) of the display device is not so large. Accordingly, it is possible to properly form wire lines on the PES substrate because deformation caused in the array process (S3) is lessened.
After the heat treatment process (S1), an inorganic material, such as SiNx, is coated on the PES substrate to form a barrier layer by a barrier forming process (S2). The barrier layer prevents an etching gas or a chemical material, such as an etchant or a stripper, used in the array process (S3) from penetrating the PES substrate to deform the PES substrate.
Each wire line of the display device may be formed by the array process (S3) on the PES substrate after the above mentioned barrier layer is formed.
However, utilizing PES as the material for the flexible substrate becomes very expensive, thereby increasing manufacturing costs of the display device. That is, the PES substrate is an expensive plastic material.
On the other hand, a plastic such as polyethylene naphthalate (PEN) is relatively inexpensive and easy to fabricate. However, PEN tends to have a characteristic directivity of the fabrication process. That is, a plastic substrate made of PEN has a tendency to change in one-direction in which heat is applied to the substrate.
Thus, it is difficult to properly form a wire line in the array process because a PEN plastic substrate is non-uniform and deformed by heat generated during the process that causes the above mentioned directivity. That is, an inexpensive plastic, such as PEN, obtains a non-uniform surface roughness during an initial heat process. This causes difficulty in correctly forming a wire line in the subsequent array process. As a result, it is difficult to carry out a stable array process on plastics substrates made of materials other than PES. That is, it is difficult to carry out a stable array process on a plastic substrate, such as PEN, which is inexpensive and easy to fabricate.